Nitrogen's Role in Changing Kernel Weight in Maize: Relevant Physiological Mechanisms During Reproductive Stages

Lia Belén Olmedo Pico, Purdue University

Abstract

Although grain yield (GY) in maize (Zea maysL.) is the product of both kernel number (KN) per unit area and kernel weight (KW), the latter has historically been considered the least variable component. However, as a result of sink strength enhancement by genetic improvement, KW in modern genotypes has become more responsive to changes in environmental and management conditions. Furthermore, KW has recently been proven to play a bigger role in the genetic improvement of maize GY -in temperate U.S. germplasm- than it had before. Therefore, the prospect of KW becoming a more important driver behind GY variability warrants embarking on more intensive research into the physiological mechanisms underlying when post-flowering stress conditions can limit KW. In pursuit of that goal, this dissertation evaluated the effects of N availability on: 1) the sources of dry matter (DM) and N assimilates for the growing kernels during the reproductive period; 2) the determination of potential KW (i.e., potential kernel sink capacity) during the lag phase of grain filling; and 3) the realization of final KW (i.e., actual sink capacity) during the linear phase of grain filling. To investigate how N availability affected source capacity during reproductive growth, a 2- year field study that combined N timing and N rate treatments (under a common density of 8.3 plants m-2 ) was conducted at the Purdue Rice Farm (LaCrosse, IN) in 2016 and 2017. The timing applications included: all N applied at planting, split application between planting and early sidedress, and split application between planting and V12 (for the last 56 kg N ha-1 ). The five N rates tested were 0, 112, 168, 224, and 280 kg N ha-1 . Biomass samples of plant components at R1, R3, and R6 enabled DM and N sources (i.e., post-silking DM production, post-silking N uptake, DM and N remobilization) to be calculated separately for the two main grain-filling phases: from R1 to R3 (i.e., lag phase) and from R3 to R6 (i.e., linear phase). Since N timing and its interaction with N rate had no impact on the majority of evaluated parameters, the much larger N rate impacts were therefore averaged across N timing treatments. In both seasons, lag-phase DM production (PostDMR1.R3) was much less responsive to N rates than that during the linear-phase (PostDMR3.R6). In the lag phase, substantial DM gains in leaves and stems occurred in both years, while N content gains were mostly detected in reproductive tissues. Differential seasonal patterns in post-silking N uptake were observed, with plants either achieving net above-ground N content gains only during the lag phase (PostNR1.R3) in 2016, or during the linear phase (PostNR3.R6) in 2017). Both GY and KW were gradually increased by N supply, with reproductive tissues proving to be relatively stronger sinks for N than for DM during the lag phase. To understand how N availability affected the determination of potential KW, three field experiments testing N rates, plant densities and N timing applications on a single commercial hybrid (DKC63-60) were conducted over a 3-year period.

Degree

Ph.D.

Advisors

Vyn, Purdue University.

Subject Area

Nutrition|Physiology|Cellular biology|Developmental biology|Meteorology|Plant sciences

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